The invention relates to a method for producing pig iron, in which iron ore is reduced to sponge iron in a reduction shaft and this sponge iron is then introduced into the head of a melting gasifier and melted there with the aid of a gasification agent.
In a plant for producing pig iron from iron ore, in which a reduction shaft and a melting gasifier are coupled to one another, during the charging of the melting gasifier with the reduced iron carriers and calcined loading materials only via the reduction shaft and with carbon carriers via the coal line, it is only possible in very few cases and for quite short periods of time to operate both units without their influencing one another in a negative manner. The two units are operationally connected to one another in such a way that normally the operation of the entire plant is shifted alternately for a period in favour of the reduction shaft and then for a period in favour of the melting gasifier. For a period of time, sufficient reduction gas is thus produced in the melting gasifier, by which means the reduction shaft is operated with a high specific amount of reduction gas and the iron carriers and loading materials, which are charged from the reduction shaft into the melting gasifier have a high degree of metallisation and calcination. Consequently, the energy requirement in the melting gasifier is lower, the temperatures of pig iron, slag and/or dome rise, the amount of oxygen is lessened and the amount of reduction gas produced in the melting gasifier becomes smaller. In turn, an inadequate amount of reduction gas for the reduction shaft results in a lower degree of metallisation and calcination of the charge introduced into the melting gasifier such that the energy requirement in the melting gasifier rises again, the temperature level in the melting gasifier drops, the specific amount of oxygen for the gasifier is increased and the new cycle begins again afresh with a higher input of energy. Only through the selection of raw materials which are matched to one another and by the early introduction of countermeasures is it possible to avoid the formation of such cycles with all their negative consequences such as pig iron which is too cold or too hot, fluctuations in the silicon, carbon and sulphur content of the pig iron, and the like. In order to produce usable pig iron continuously, the plant is operated with a much higher specific energy consumption and with a much greater fluctuation in the quality of the pig iron than would be the case if the two units were operationally uncoupled from one another.
From this arises the object of the present invention which consists in operationally uncoupling the two units from one another and operating them without their influencing one another in a negative manner.
This object is accomplished according to the invention by having a high degree of metallisation of over 90% of the sponge iron introduced into the melting gasifier, and introducing iron oxide into the melting gasifier to reduce the degree of metallization. Additional developments of the method according to the invention are described hereinbelow.
Through the combined charging, matched to one another, of the melting gasifier with a well reduced sponge iron, i.e. one with a high degree of metallisation, from the reduction shaft, and with iron oxide preferably via the gasification agent line, there are supplied to the melting gasifier overall iron carriers with a controlled metallisation, which is relatively independent of the metallisation of the sponge iron alone, such that the operation of the melting gasifier is optimised. Through the direct introduction of the iron oxide via the head of the melting gasifier, additional oxygen is supplied to the latter and reacts with fine particles of gasification agent in the dome of the melting gasifier, exploiting the surplus amount of heat. In order to avoid a lack of carbon in the dome of the melting gasifier and to counteract the oxidation, by excess oxygen, of CO to form CO2, in addition to iron oxide, gasification agent undersize is also supplied to the melting gasifier via the gasification agent line. Through a melting gasifier which is balanced in terms of energy and through an adequate amount of reduction gas for the reduction shaft, both units are thus operated without influencing one another.
A high and uniform metallisation of the sponge iron from the reduction shaft gives sufficient latitude for the latter to be adapted, through the addition of the iron oxide, to the excess energy in the upper region of the melting gasifier and to the quality of the raw materials, particularly of the gasification agent.
The degree of metallisation of the sponge iron from the reduction shaft should be maintained above 90%, preferably above 92%, and reduced by the addition of the iron oxide to roughly 88% or to an even lower value, if excess gas production is economically viable (e.g. for producing sponge iron or generating power). In this case, introducing gasification agent undersize and blowing increased amounts of oxygen into the dome of the melting gasifier makes sound technical and economic sense.
In order to be able to use all the iron oxide occurring during the sifting of the iron carriers and a portion of the gasification agent undersize, the degree of metallisation of the sponge iron from the reduction shaft and the amount of oxygen for the dome of the melting gasifier are correspondingly adapted.
The increased energy requirement for the reduction of the iron oxide undersize and the gasification of the gasification agent undersize in the upper region of the charge bed and in the dome region of the melting gasifier are covered by blowing increased amounts of oxygen into the dome of the melting gasifier.
Undersize containing carbon is supplied to the melting gasifier via the gasification agent line in order to cover the increased requirement for carbon carriers in the dome region of the melting gasifier and to prevent the combustion of CO into CO2.
Through the additional input of oxygen in the form of iron oxide, volatile components of the gasification agent undersize and gaseous oxygen into the upper region of the melting gasifier, an adequate amount of reduction gas is produced in the melting gasifier, and thus the reduction shaft can be operated with a high and uniform specific amount of reduction gas, independently of the proportion of volatile constituents of the gasification agent.
By uncoupling the operation of the reduction shaft in this way from the operation of the melting gasifier what is achieved is that these two main units can be operated without influencing one another.